This application is based upon and claims priority from prior French Patent Application No. 98-16383, filed Dec. 21, 1998, the entire disclosure of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to electronic circuits, and more specifically to electromagnetic transponders that are interrogated contactless and wireless by a read/write terminal.
2. Description of Related Art
Conventional electromagnetic transponders are transceivers that typically lack an independent power supply, and instead extract the power required by the electronic circuits included therein from a high frequency field radiated by the antenna of the read/write unit. Such electromagnetic transponders are based on the use of oscillating circuits, on the transponder side and on the read/write unit side. These circuits are coupled by a close magnetic field when the transponder enters the field of the read/write unit. The range of a transponder system, that is, the maximum distance from the terminal at which a transponder is activated (awake) depends, especially, on the size of the transponder antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, and on the intensity of this excitation.
FIG. 1 schematically shows a conventional data exchange system between a read/write unit 1 and a transponder 10. Generally, unit 1 is formed of an oscillating circuit formed of an inductance L1 in series with a capacitor C1 between an output terminal 2 of an antenna coupler 3 and a terminal 4 at a reference potential (generally, ground). Coupler 3 receives a signal f1, provided by an oscillator 5, that forms a high frequency carrier. Signal f1 is used in the absence of a data transmission from terminal 1 to transponder 10, as an xe2x80x9cenergy sourcexe2x80x9d for activating transponder 10 if the transponder enters the field. If necessary, a modulator 6 provides a data signal based on data, received from an input e that is the output of an electronic system (not shown).
The junction point of capacitor C1 and inductance L1 forms, in the example shown in FIG. 1, a terminal rx for sampling a data signal, received from a transponder 10 and intended for a demodulator 7. An output s of the demodulator communicates the data received from transponder 10 to the electronic system of the read/write unit. Demodulator 7 receives, from oscillator 5, a signal f2 (most often, with the same frequency as signal f1) to enable the demodulation. The demodulation may be performed from a signal sampled across antenna coupler 3 and not across inductance L1. By the excitation from signal f1 inductance L1 of unit 1 generates a high frequency field of small intensity. On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a reception resonant circuit) for capturing the field generated by the oscillating circuit of unit 1. The resonant circuit (L2-C2) of transponder 10 is tuned on the frequency of the oscillating circuit (L1-C1) of unit 1.
Terminals 11 and 12 of the resonant circuit (corresponding to the terminals of capacitor C2) are connected to two AC input terminals of a rectifying bridge 13 that is formed, for example, of four diodes D1, D2, D3, and D4. In the circuit of FIG. 1, the anode of diode D1 and the cathode of diode D3 are connected to terminal 11. The anode of diode D2 and the cathode of diode D4 are connected to terminal 12. The cathodes of diodes D1 and D2 form a positive rectified output terminal 14. The anodes of diodes D3 and D4 form a reference terminal 15 of the rectified voltage. A capacitor Ca is connected in parallel with rectified output terminals 14 and 15 of bridge 13 to filter the rectified voltage provided by the bridge.
When transponder 10 is in the field of unit 1, a high frequency voltage is generated across the resonant circuit. This voltage, rectified by bridge 13 and smoothed by capacitor Ca, provides a supply voltage Va to electronic circuits of the transponder via a voltage regulator 16. The electronic circuits of the transponder have been symbolized in FIG. 1 by a block 17. This block P generally is a chip (most often integrating regulator 16) containing at least one memory and one processor. To transmit data from transponder 10 to unit 1, block 17 controls a stage of modulation (back modulation) of the resonant circuit (L2-C2).
This modulation stage is generally formed of an electronic switch (a transistor 18) and a resistor R, associated in series between terminals 14 and 15. Transistor 18 is controlled at a much smaller frequency (generally by a factor of at least 10) than frequency f1 of the excitation signal of the oscillating circuit of unit 1. The oscillating circuit of the transponder is thus submitted to an additional damping as compared to the load formed of regulator 16 and circuit 17 when switch 18 is closed. The voltage decreases across winding L2 so that the transponder takes a greater amount of energy from the high frequency field.
In the system range, two effects of the presence of the transponder in the field can be made out. At a relatively large distance (that is, substantially at the range limit), the oscillating circuit of the terminal operates with no interference. At a smaller distance, there is an increase in the charge of the oscillating circuit of unit 1. Accordingly, the amplitude across inductance L1 decreases. Thus, there is a transfer of the amplitude modulation performed by transponder 10 to read/write unit 1. which can then detect the presence of transponder 10 in its field. Another method consists of detecting the phase variation due to this change of charge on the oscillating circuit of unit 1.
Different types of modulation may be used for the data exchanges between unit 1 and transponder 10. Most often, an amplitude modulation representing either the entirety of signal f1 (all or nothing modulation), or a small portion (on the order of 10%) of this amplitude is used, according to the supply need of transponder 10. It should be noted that, whatever the type of modulation used (for example, phase or frequency modulation) and whatever the type of data coding (NRZ, NRZI, or Manchester), the transmission of the modulation is performed digitally, by skip between two binary levels.
The matching frequency of the oscillating circuits conditions the transmission rate since the frequency of modulation, by switch 18 on the transponder side must be clearly smaller than the carrier frequency used to supply the transponders. Accordingly, the higher the supply carrier frequency, the greater the data flow rate can be. For example, conforming to a standard ISO 14443 of small distance transponder systems (distance smaller than twenty centimeters), the frequency of carrier f1 is 13.56 MHZ and the frequency of the control pulses of switch 18, on the transponder side, is 847 kHz (16 times less).
Conventional transponder systems such as those described hereabove suffer from several drawbacks. A first drawback is that xe2x80x9ctransmission gapsxe2x80x9d, that is, distances between the transponder and the terminal at which the terminal does not detect the transponder even though the transponder is in its field, can occur. Such transmission gaps occur when the transponder is very close to the read/write unit, that is, when the distance between both inductive coupling elements L1 and L2 is small as compared to the system operating range. For example, for an application to small distance systems under a 13.56 MHZ frequency, the range is on the order of 10 centimeters and detection losses appear when the transponder is at less than three centimeters from the read/write unit. A conventional solution for overcoming this problem is to force a minimum interval between the transponder and unit 1. However, a disadvantage of such a solution is that is reduces the system range by this interval.
Another drawback of conventional transponder systems is that the use of a resistor R in the modulation stage of transponder 10 causes a dissipation upon modulation by the transponder. Because the transponder is not independently supplied but takes its power supply from the high frequency field coming from the read/write system, power consumption must be minimized.
In view of these drawbacks, it is an object of the present invention to overcome the above-mentioned drawbacks and to provide a transponder that avoids transmission gaps while maintaining the maximum system range. On the transponder side, a modulation element acts upon the resonance frequency of the oscillating circuit and not upon its amplitude, as in the case of a resistor. Thus, there is a slight detuning of the oscillating circuits in a modulation (back modulation) by the transponder.
Another object of the present invention is to provide a transponder that is simple and inexpensive with respect to the components used, to help the transponder miniaturization.
Yet another object of the present invention is to reduce the power consumption of a transponder.
Still another object of the present invention is to provide simple circuitry to enable modification of the equivalent capacitance of the oscillating circuit. For example, simple MOS transistors can be provided to enable the control of a capacitive modulation circuit of a transponder oscillating circuit.
One embodiment of the present invention provides an electromagnetic transponder that includes an oscillating circuit, an electronic circuit, a rectifying circuit, and a capacitive modulation circuit. The oscillating circuit includes an inductive element and the electronic circuit includes a transmission circuit for transmitting digitally-coded information. The rectifying circuit is coupled to the oscillating circuit to provide a DC supply voltage to the electronic circuit, and the capacitive modulation circuit is coupled to both end terminals of the inductive element and to the reference potential of the electronic circuit. In a preferred embodiment, the capacitive modulation circuit includes two capacitors, with capacitor being coupled between one end terminal of the inductive element and the reference potential and the other capacitor being coupled between the other end terminal of the inductive element and the reference potential.
Another embodiment of the present invention provides a system of electromagnetic transmission that includes at least one transponder and a terminal that generates an electromagnetic field for communicating with the transponder when the transponder is in the electromagnetic field. The transponder includes an oscillating circuit, an electronic circuit, a rectifying circuit. and a capacitive modulation circuit. The oscillating circuit includes an inductive element and the electronic circuit transmits digitally-coded information to the terminal. The rectifying circuit is coupled to the oscillating circuit to provide a DC supply voltage to the electronic circuit, and the capacitive modulation circuit is coupled to both end terminals of the inductive element and to the reference potential of the electronic circuit. In one preferred embodiment, the transponder also includes a resistive modulation circuit coupled in parallel with a capacitor to filter the DC supply voltaage provided by the rectifying circuit.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, arc given by way of illustration only and various modifications may naturally be performed without deviating from the present invention.